This invention relates to a photosensitive composition which can give highly heat-resistant relief structures.
There is an increasing demand for photochemically crosslinkable polymers functioning as insulating, passivation or protective coatings in semiconductor devices. Such film patterns must be resistant to heat and aging while being highly insulating. Frequently a short-term temperature stability of up to 450.degree. C. is required together with an operating temperature of up to 200.degree. C. These requirements cannot be met with current photoresist formulations.
There are a number of soluble precursors which can be converted into highly heat-resistant final products by thermal ring closure reactions. Well known examples of such resistant polymers are the aromatic polyimides developed by DuPont, which have been utilized for years as varnishes and films in the electric and electronics industries.
Methods are known for making relief patterns of highly heat resistant polymers.
Analogous to a metal substrate etching technique, Epifano and Jordan produced patterns of soluble polyimide precursors by use of phototechniques with conventional positive resists. The relief patterns were subsequently converted to polyimide form by heat curing: See German Auslegeschrift DAS No. 1,764,977. In a similar way Jones and Agnihotri produced polyimide film patterns via phototechniques by etching polyimide layers with hydrazine and ethylenediamine respectively: see J. I. Jones, J. Polymer Sci., part C, (22) 773 (1969) and R. K. Agnihotri, Proc. SPE Regional Tech. Conf. Ellenville, N.Y. Oct. 13-15, 74, (1976).
The indirect method of production of film patterns above mentioned is complicated by the additional required steps of producing and removing the photoresist coating. Well defined boundaries between soluble and insoluble regions cannot be produced within the layer of the polyimide precursor.
Photosensitive polyimide precursors are also known. They allow a direct production of film patterns of heat resistant polymers of photochemical means.
Kerwin and Goldrich photosensitized polyimide precursors by the addition of dichromate: see Polymer Engineering and Science 11, 426 (1971). However, their application as a negative photoresist, which is annealed after exposure and development for the preparation of highly heat resistant relief patterns, is limited by the fact that the solution and films of these photosensitized polyaminocarboxylic acids are very unstable and accordingly must be processed immediately after preparation. Furthermore, the highly heat resistant polyimides prepared from the sensitized substances have a considerable content of inorganic salts, which limits their possible applications, particularly as dielectrics in the electronics industry.
It is further known that polyamidecarboxylic acids can be photosensitized by incorporating photosensitive organic residues which are bound to carboxylic groups in the ester fashion: see U.S. Pat. Nos. 3,957,512 and 4,040,831. These photosensitive polyamidecarboxylic acids are also negative working. A typical example of these photosensitive polyamidecarboxylic acids is produced by the following steps:
1st step: Preparation of diallyl ester of pyromellitic acid (X.sub.1) from pyromellitic dianhydride. PA1 2nd step: preparation of diallyl ester of pyromellitic acid dichloride (X.sub.2) from X.sub.1 and thionyl chloride. PA1 3rd step: preparation of a polyimide precursor (X.sub.3) from X.sub.2 and diaminodiphenylether. PA1 4th step: preparation of a photosensitive polyimide precursor solution from X.sub.3, solvent and photoinitiator. PA1 (1) the mol ratio of the compound [III]to the repeating units of the polymer [I]including repeating units [II]and repeating units [IV]must be equal to or greater than 0.05, preferably equal to or greater than 0.3. PA1 (2) The mol ratio of the compound [III]to COOR.sub.3 in the polymer [I]must be equal to or less than 2.0. The compositions which satisfy these conditions have good photosensitivity and are responsive to development.
In the 3rd step, the photosensitive polymer X.sub.3 is purified by precipitation. As a result of this precipitation purification, the photosensitive prepolymer X.sub.3 can be stored in a solid state and has good storage ability. However, the methods of these patents for preparing the photosensitive polyamidecarboxylic acids solution are complicated. The manufacturing cost of the photosensitive compositions is pushed up by these complicated methods.